The invention relates to power transmission belts, but more particularly, the invention relates to an endless tensile member for power transmission belts.
Textile cords have been used as a tensile member in power transmission belts. Cord bundles for early belts were spun or twisted from staple fibers such as cotton. The tensile strength of such cord bundles is greatly influenced by the frictional forces achieved by twisting the staple fibers together. In many cases, the cord bundles with a greater tensile strength also had longer staple fiber lengths. The prickly ends of the fibers extending from the cord bundles formed a mechanical bond with the polymeric material of the belt which improved the tension carrying capacity of the cord by increasing the force at which the fibers would slip relative to each other.
The load carrying capacity of power transmission belts was greatly improved with the advent of cord bundles made from synthetic yarns of continuous multi-filaments. It was and currently is theorized that improved belt performance results from the substitution of staple fibers with filaments that extend continuously throughout the length of the belt to substantially eliminate slippage between filaments forming the cord bundle.
While the multi-filament synthetics greatly improved the load carrying performance of a power transmission belt, the early synthetics introduced some problems. Synthetic materials such as rayon were produced in multi-filament fashion and then purposely chopped, combed, oriented and twisted into a synthetic staple cord bundle. The fibers of this synthetic staple were purposely made longer to reduce slip between the twisted fibers. Although the twisted staple cord bundle did not have the tensile strength of its corresponding multi-filament counterpart, belts produced with such cord bundles were smoother running because of a reduced cord modulus.
For example, ten belts with a staple rayon cord bundle tensile member and ten belts with a multi-filament rayon cord bundle tensile member were built and tested. Except for the tensile members, the belts had substantially the same constructions. In accelerated tests, the belts with the multi-filament rayon tensile member had average lives of 195 hours whereas the belts with the staple rayon tensile member had average lives of 48 hours.
Newer synthetic fibers such as nylon and polyester have a more satisfactory modulus which eliminates the need for producing staple cords for belts where shock loading is a problem. The new synthetic fibers lead the way for producing belts of even higher load carrying capacity than was formerly achieved. Most power transmission belts presently produced use cord bundles of the continuous multi-filament type.
U.S. Pat. No. 3,855,870 to Schnackenberg discloses a cord bundle, a combination of continuous multi-filament and pseudo staple fibers for use with power transmission belts. The cord is produced by disassociating a plurality of end portions from a multi-filament cord bundle. Improved belt performance is believed to result from improved mechanical bonding of the tensile member with the polymeric material of the power transmission belt. Nevertheless, it is generally believed that a multi-filament cord bundle with a predominate number of continuous filaments is better as a tensile member than a cord bundle of twisted staple fibers.
The present art also uses adhesives and chemical agents to further enhance bonding between the polymeric material of a belt and the tensile member of spirally wound cord. Thus, the present belt art takes advantage of continuous multi-filament cord bundles and chemical agents or adhesives to effect power transmission belts of high performance.
Perhaps one reason why belts with staple fiber cord bundles do not have the equivalent load carrying capacity as belts having a multi-filament cord bundle, is that the individual staple fibers are stressed past their yield point when power is transmitted. For example, assume that two such staple fibers are twisted together. If the fibers are tensioned, each end of the fiber will be unstressed while the center portions of the fiber are stressed. The tensile load stresses must be less than that force which is required to separate adjacently twisted fibers.
Next assume that two staple fibers in pseudo being parallel and twisted together at their end portions where the center portion of one fiber is taut while the center portion of the fiber is slackened. When a tensile load is applied to the fibers, the taut fiber will be overstressed while the slackened fiber will have zero stresses. If this tension load is that which is required to transmit power with a V-belt, the taut fiber may become over-stressed and break. A cord bundle of twisted, staple fibers will have a multitude of fibers in such an arrangement.
When staple fibers are adhered together such as with polymeric material of a power transmission belt, they do not easily slip relative to one another to give each fiber its proportional share of a tensile load. The originally taut fibers may be relatively over-stressed so that slack will be taken from the slackened staple fibers so they can share in transmitting a load. Thus, some fibers will be highly stressed whereas other staple fibers of the cord bundle will be lowly stressed.
Belt life may be increased in some drives by lowering the modulus of the tensile member. Generally, the type of drive where increased belt life may be accomplished is those drives where small diameter sheaves are used. An example of such a drive is a front end automotive drive. Belt life may be increased by lowering the modulus provided that the reduction in stresses achieved by lowering the modulus is greater than the strength loss resulting from the change to a lower modulus material.
A reduction in modulus may be achieved in generally two ways. First, a lower modulus material may be used for the multi-filament cord bundle. Any gain in belt life because of a lower modulus may be predicted.
Secondly, a staple cord bundle may be made from a synthetic fiber of substantially the same modulus. When this is done the modulus of the cord bundles is reduced. Heretofore, the loss in modulus from use of staple fibers has always been greater than the reduction in stress concentrations that may have been gained. It is believed that this is because of the parallel filament loading concept discussed above. Thus, prior art power transmission belts with staple fibers have not had as good of belt life as power transmission belts built with multi-filaments of the same material.